https://www.youtube.com/watch?v=Rpj0emEGShQ TEKS 4.C Students will… Compare the structure of viruses to cells, describe viral reproduction, and describe the role of viruses in causing diseases such as HIV and influenza

https://www.youtube.com/watch?v=Rpj0emEGShQ About Viruses A virus is a non-cellular, non-living particle that invades and then multiplies inside a living cell Smaller than the smallest living cell

About Viruses

About Viruses The only way that viruses are like other organisms is that they can multiply – but only by getting energy from a host A host is the living organism that the virus multiplies inside of Viruses act like parasites – organisms that live on or in a host and cause it harm Almost all viruses destroy their host cells

About Viruses All viruses have 2 basic parts A capsid (protein coat) that protects the virus An inner core of RNA or DNA that contains the instructions for making new viruses Eukartyotic viruses usually have protective envelopes made from the host cell membrane

Virus Attachment Each virus contains unique receptor proteins on its surface The shape of the protein allows the virus to attach to certain cells in the host. This “lock-and-key” system allows certain viruses to attach to specific host cells Example: most cold viruses infect cells only in the nose and throats of humans

Virus Attachment

Bacteriophages Viruses that attack bacteria T-phages are a specific class that have icosahedral heads, double-stranded DNA, and tails Six spikes at the base of the contractile tail are used to attach to the host cell and inject the viral DNA

Bacteriophages

Retroviruses Contain RNA, not DNA Contain an enzyme called reverse transcriptase that is injected into the cell along with the RNA Reverse transcriptase causes synthesis of a complementary DNA molecule (cDNA) using the virus RNA as a template Examples: HIV and influenza

Retroviruses

Lytic Cycle Once inside a cell, the genetic material of the virus takes over many of the cell’s functions The genetic material instructs the cell to reproduce the virus Active viruses take over the cell functions immediately (lytic cycle) Examples: colds and the flu Takes a few days to feel sick

Lytic Cycle Five steps Attachment to the cell Injection of viral genetic material (DNA or RNA) Replication (biosynthesis) of new viral proteins and nucleic acids Assembly (maturation) of the new viruses Release of the new viruses into the environment (cell lyses)

Lytic Cycle

Lytic Cycle

Lysogenic Cycle Some viruses have the ability to become dormant once they are inside the cell They are called latent viruses They can remain inactive for very long periods of time (years) Later, they activate and enter the lytic cycle in response to some external signal Examples: HIV and herpes

Lysogenic Cycle Seven steps Attachment to the cell Injection of viral genetic material (DNA or RNA) Virus genetic material integrates into host cell genetic materia to hide An external signal activates the viral genetic material, which separates from the host cell genetic material Replication (biosynthesis) of new viral proteins and nucleic acids Assembly (maturation) of the new viruses Release of the new viruses into the environment (cell lyses)

Lysogenic Cycle

Reproduction Comparison

Spreading of viruses Contact with a contaminated object Droplets of moisture in the air (cold and flu viruses) A bite from an infected animal (rabies) Exchange of bodily fluids (ex: blood, semen, or vaginal secretions) Vectors – an organism that transmits a disease without being affected by that disease (ex: mosquitos) https://www.youtube.com/watch?v=R7GXAxnfiq8

Treating Viruses No cures currently exist for viral diseases The best treatment for a virus is a lot of rest, drinking plenty of fluids, and well-balanced meals Over-the-counter medications can help to relieve symptoms Some viruses can be treated with anti-viral drugs (anti-virals do NOT destroy the virus, simply inhibit development) Antibiotics do NOT help in getting over a virus

Preventing Viral Disease Vaccine: a substance that is introduced into the body which causes the body to produce chemicals that kill the virus (immune response) Vaccines are made from either weakened or heat-killed viruses (attenuated) The killed virus does not cause the viral disease, but it still causes the body to produce chemicals that destroy the virus The body is able to “remember” the virus so that if it invades again it will be immediately destroyed

Influenza Virus (Flu)

Influenza

Influenza Retrovirus (RNA) Lytic Spread via droplets (sneezing, coughing, etc) Yearly vaccines due to the rate of mutation that changes the surface proteins of the virus (so they are no longer recognized by the immune system)

Human Immunodeficiency Virus (HIV)

HIV

HIV Retrovirus (RNA) Spreads through exchange of bodily fluids (blood; semen and vaginal secretions) Lysogenic Destroys helper T-cells of the immune system, which causes the immune system be unable to defend against pathogens The virus is HIV; the disease is AIDS (autoimmune deficiency syndrome)